When Should I put my Baby Chicks out... it is winter...

Last spring I got five BR's at the feed store. When they got to be five weeks old, I put them out in their coop.

If it was spring, I would put these 10 out when they got to be five weeks of age, however it's not spring. It's Northern PA and cold in the winter. My coop has a red heat lamp in it, but isn't insulated. It has a wood floor and such, but since they will be young yet, how much longer should I wait until I put them out in their coop?